4. 4 Bits Isn’t Enough

The original 4004 microprocessor chip handled data in four bit chunks. Four bits gives you sixteen possible numbers, enough to handle standard decimal arithmetic for a calculator. If it were only the size of the numbers we calculate with, we might still be using four bit microprocessors.

The problem is that there is another form of calculation a stored instruction computer needs to do. That is it has to figure out where in memory instructions are. In other words, it has to calculate memory locations to process program branch instructions or to index into tables of data.

Like I said, four bits only gets you sixteen possibilities and even the 4004 needed to address 640 bytes of memory to handle calculator functions. Modern microprocessor chips like the Intel Pentium 4 can address 18,446,744,073,709,551,616 bytes of memory, though the motherboard is limited to less than this total. This led to the push for more bits in our microprocessors. We are now on the fence between 32 bit microprocessors and 64 bit monsters like the AMD Athlon 64.

5. The First Step Up, 8 Bits

With a total memory address space of 640 bytes, the Intel 4004 chip was not the first microprocessor to be the starting point for a personal computer. In 1972, Intel delivered the 8008, a scaled up 4004. The 8008 was the first of many 8- bit microprocessors to fuel the home computer revolution. It was limited to only 16 Kilobytes of address space, but in those days no one could afford that much RAM.

Two years later, Intel introduced the 8080 microprocessor with 64 Kilobytes of memory space and increased the rate of execution by a factor of ten over the 8008. About this time, Motorola brought out the 6800 with similar performance. The 8080 became the core of serious microcomputers that led to the Intel 8088 used in the IBM PC, while the 6800 family headed in the direction of the Apple II personal computer.

6. 16 Bits Enables the IBM PC

By the late '70s, the personal computer was bursting at the seams of the 8 bit microprocessor performance. In 1979, Intel delivered the 8088 and IBM engineers used it for the first PC. The combination of the new 16 bit microprocessor and the name IBM shifted the personal computer from a techie toy in the garage to a mainstream business tool.

The major advantage of the 8086 was up to 1 Megabyte of memory addressing. Now, large spreadsheets or large documents could be read in from the disk and held in RAM memory for fast access and manipulation. These days, it’s not uncommon to have a thousand times more than that in a single 1 Gigabyte RAM Module, but back in that time it put the IBM PC in the same league with minicomputers the size of a refrigerator.

7. Cache RAM, Catching Up With the CPU

We’ll have to continue the march through the lineup of microprocessors in the next installment to make way for the first of the enhancements that you should understand. With memory space expanding and the speed of microprocessor cores going ever faster, there was a problem of the memory keeping up.

Large low-powered memories cannot go as fast as smaller higher power RAM chips. To keep the fastest CPUs running full speed, microprocessor engineers started inserting a few of the fast and small memories between the main large RAM and the microprocessor. The purpose of this smaller memory is to hold instructions that get repeatedly executed or data that is accessed often.

This smaller memory is called cache RAM and allows the microprocessor to execute at full speed. Naturally, the larger the cache RAM the higher percentage of cache hits and the microprocessor can continue running full speed. When the program execution leads to instructions not in the cache, then the instructions need to be fetched from the main memory and the microprocessor has to stop and wait.